One type of nonvolatile memory known in the art relies on magnetic memory cells. Known as magnetic random access memory (MRAM) devices, these devices include an array of magnetic memory cells. The magnetic memory cells may be of different types. For example, a magnetic tunnel junction (MTJ) memory cell or a giant magnetoresistive (GMR) memory cell.
The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization may be referred to as a sense layer or data storage layer and the magnetic film that is fixed may be referred to as a reference layer or pinned layer.
Conductive traces (commonly referred to as word lines and bit lines, or collectively as write lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. Located at each intersection of a word line and a bit line, each memory cell stores the bit of information as an orientation of a magnetization. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a matching (i.e., parallel) or opposing (i.e, anti-parallel) orientation with respect to the orientation of magnetization in the reference layer, depending on the desired logic state.
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logical values of “1” and “0”. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line crossing the selected memory cell. The currents create magnetic fields that, when combined, can switch the orientation of magnetization of the selected memory cell from parallel to anti-parallel or vice versa.
FIGS. 1a through 1c illustrate the storage of a bit of data in a single memory cell 20. In FIG. 1a, the memory cell 20 includes an active magnetic data film 22 and a pinned magnetic film 24 which are separated by a dielectric region 26. The orientation of magnetization in the active magnetic data film 22 is not fixed and can assume two stable orientations as shown by arrow M1. On the other hand, the pinned magnetic film 24 has a fixed orientation of magnetization shown by arrow M2. The active magnetic data film 22 rotates its orientation of magnetization in response to electrical currents applied to the write lines (130,132, not shown) during a write operation to the memory cell 20. The first logic state of the data bit stored in memory cell 20 is indicated when M1 and M2 have matching (i.e, parallel) orientations as illustrated in FIG. 1b. For instance, when M1 and M2 have matching orientations, a logic “1” state is stored in the memory cell 20. Conversely, a second logic state is indicated when M1 and M2have opposite (i.e, anti-parallel) orientations as illustrated in FIG. 1c. When the orientations of M1 and M2 are opposite each other, a logic “0” state is stored in the memory cell 20. In FIGS. 1b and 1c the dielectric region 26 has been omitted. Although FIGS. 1a through 1c illustrate the active magnetic data film 22 positioned above the pinned magnetic film 24, their positions may be reversed.
The logic state of the data bit stored in the memory cell 20 can be determined by measuring its resistance. The resistance of the memory cell 20 is reflected by a magnitude of a sense current 23 (referring to FIG. 1a) that flows in response to read voltages applied to the write lines 30, 32.
In FIG. 2, the memory cell 20 is positioned between the write lines 30, 32. The active and pinned magnetic films 22,24 are not shown in FIG. 2. The orientation of magnetization of the active magnetic data film 22 is rotated in response to a current Ix that generates a magnetic field Hy and a current Iy that generates a magnetic field Hx. The magnetic fields Hx and Hy act in combination to rotate the orientation of magnetization of the memory cell 20.
As illustrated in the above Figures, the layers of magnetic material are typically formed as geometrically patterned films such as squares ellipses, or rectangles. One disadvantage of patterned magnetic layer storage structures is that patterned magnetic layers generate a magnetostatic field that tends to demagnetize the layer. This demagnetizating field tends to reorient the magnetization of the thin film so as to minimize the energy of the patterned element, the end result being a non-uniform or multi-domain magnetization state. Magnetostatic fields from patterned layers also interact with magnetic material in proximity to the edges of the patterned film, potentially disrupting the magnetization state in the proximate magnetic material. For example, referring to FIG. 1a, the magnetization M2 of pinned magnetic film 24 creates a demagnetization field in a direction opposing M2. This field interacts with data film 22 and biases the magnetic hysteresis loop of the data film such that the hysteresis loop may no longer be symmetric about zero field. In a memory application this offset can be very damaging. If the offset is greater than the coercivity of the data film, then there is loss of data after removal of the writing field. An offset field lower than the coercivity is also detrimental in that it introduces asymmetry into the writing process. Any variation in this offset field adversely affects the writing margin when attempting to write a single data film within an array of memory elements.
When reading the magnetic memory elements, non-uniform magnetization or multiple domains tend to create noise or areas of varying resistance across the memory cell that makes determination of the state of the memory cell difficult or impossible. In addition, variation in the domain states can produce fluctuations in the switching field that can render the memory cell writing process unpredictable. From the above, it can be seen that maintaining a uniform magnetization direction in the magnetic layers is important. In the case of the fixed magnetization of the reference layer, it is thus desirable to pin the magnetization in a manner that minimizes the presence of magnetostatic fields that may interact in a deleterious manner with the data film.